This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. ABSTRACT: The mitochondrial IM is a very flexible membrane that can respond rapidly to changes in osmotic and metabolic conditions. However, its structural responses do not involve simple unfolding and refolding. In the matrix-contracted (condensed) state, rat liver mitochondria contain numerous dilated cristae with multiple tubular connections to each other as well as to the IM periphery. In the matrix-expanded (orthodox) conformation, the IM is more tubular with usually single peripheral junctions. This has led us to hypothesize that the morphology of the inner membrane is determined to a large extent by a balance between membrane fusion and fission (Mannella et al., 2001). We have found that certain conditions (such as oxidative stress: Deng et al, 1999, 2001) and certain proteins (such as tBid: Scorrano et al., 2002) appear to favor IM fusion. Mgm-1 is a dynamin-like protein that is required for mitochondrial fusion (van der Bliek, review). In humans, mutations in Mgm-1 cause a form of blindness (dominant optic atrophy). C. elegans mutants of Mgm-1 have abnormal mitochondria in which the cristae are detached from the peripheral IM. This suggests that Mgm-1 may stabilize the tubular cristae junctions (van der Bliek, Mannella, et al., manuscript submitted). Interestingly, detached cristae also have been observed in a mitochondrial myopathy, Senger's syndrome (M. Huizing, see Frey and Mannella, 2000), for which the molecular defect has not yet been identified. At the RVBC, we are collaborating with several groups that are generating knock-out mice or mutant cell lines, or using knock-down approaches (like RNAi) to eliminate or reduce levels of proteins thought to be involved either in dynamin-like activities or contact site formation. These include Mgm-1 (van der Bliek in C. elegans;Pain in mammalian cells);VDAC, and cyclophilin D, involved in formation of the permeability transition pore which may occur at contact sites (Forte and Bernardi). In those cases for which preliminary EM results from collaborators suggest abnormal mitochondrial structure, the RVBC will undertake in situ tomographic reconstruction of the mitochondria. These investigations will require require progress under RVBC technological development projects associated with TRD#1: use of native tissue and cells, high resolution tomography, and a membrane segmentation. References 1. Deng, Y., Marko, M., Buttle, K., Leith, A., Mieczkowski, M., and Mannella, C. A. (2001). Cubic membrane structure in amoeba (Chaos carolinensis) mitochondria determined by electron microscopic tomography. J Struct Biol 127:231-239. 2. Deng, Y., Kohlwein, S., and Mannella, C. A. (2002). Fasting induces cyanide-resistant respiration and oxidative stress in the amoeba Chaos carolinensis: Implications for the cubic structural transition in mitochondrial membranes. Protoplasma 219:160-167. 3. Mannella, C. A., Pfeiffer, D. R., Bradshaw, P. C., Moraru, I., Slepchenko, B., Loew, L. M., Hsieh, C., Buttle, K., and Marko, M. (2001). Topology of the mitochondrial inner membrane: dynamics and bioenergetic implications. IUBMB Life 52:93-100. 4. Scorrano, L., Ashiya, M., Buttle, K., Oakes, S. A., Mannella, C. A., and Korsmeyer, S. J. (2002). A distinct pathway remodels mitochondrial cristae and mobilizes cytochrome c during apoptosis. Devel Cell 2:55-67. 5. Frey, T. G. and Mannella, C. A. (2000). The internal structure of mitochondria. Trends Biochem Sci 25:319-324. In previous reporting periods, several blocks of yeast cells were received from Dr. Pain. A double-tilt tomographic reconstruction was made from a 0.25[unreadable]m thick section of the MGM1 knock-out sample, and a surface-rendered model was prepared. + Dr. van der Bleik presented a talk entitled "Mitochondrial division proteins in C. elegans and mammals" at the European Association for Vision and Eye Research Conference, October 4-7 2006, Vilamoura, Portugal. The abstract is cited in Acta Ophthalmologica Scandinavica. 84 Supplement 239:147.